That is a damned good price for an unused UPS of that capacity and pedigree. If unused, the battery will still be good but need charging.
A Smart-UPS 1000 (SMT1000) direct form APC retails for USD 430-ish. They offer a 20% discount in their Trade-UPS trade-in programme for their own Back-UPS models but you would only be able to trade in that second-hand UPS for the SMT1500 model (or larger) which they retail at USD 559.
I cannot say if the Back-UPS model would actually *damage* a MP, but I hear that they definitely elicit protests from the power supply. The issue only occurs when the UPS is running on battery. If you could take it on approval, you could check and see (once you have allowed it sufficient time to charge of course).
By the way, the noise you will hear when a good UPS charges largely goes away when the battery is full.
EDIT : the model you are looking at is discontinued, by the way - it has been replaced by the BackUPS BR1500.